1. Field of the Invention
This invention relates to a method for welding and laminating polyamides applicable to the production of various articles widely used in the national economy.
2. Prior Art
It is known that polymer (including polyamide) laminate, can be obtained using special glues or bonding chemicals as well as dry self-bonding (e.g., of polyamide foils) at high temperatures under pressure (New Linear Polymers, H. Lee, D. Stoffery and K. Neville, McGraw-Hill Book Company (1967)).
The disadvantages of these methods are the requirements for the use of complex and expensive chemical compounds and high energy consumption. Furthermore, the bonding is not always satisfactory, and the bonding compounds are often deleterious with respect to the properties of the polymer used, e.g., they lower its thermal stability. Laminating of oriented foils at high temperature by self-bonding leads to a decrease in the degree of orientation and hence to deterioration of the physico-mechanical characteristics of the laminate obtained.
It is known that polyamide-6 can be crosslinked in the amorphous regions by methoxy-methylation (methylene bridges bond together the amide groups of neighboring macrochains) (1. Arakawa, F. Nagatoshi and N. Arai, J. Polym. Sci., Polym. Lett. Ed., 6 513-516 (1968)). For this purpose, polyamide is dipped into a solution of paraformaldehyde in methanol with the addition of small amounts of potassium hydroxide and anhydrous oxalic acid. After 30 hours at 30.degree. C., the crosslinking agent penetrates into the polymer and reacts with the amide groups, making bridges of methylene groups.
Crosslinking improves the polymer strength but it has the following disadvantages. When carried out on polymer granules, the subsequent processing of the polymer will be hampered due to the rise of the viscosity of its melt. Moreover, since the penetration of the crosslinking agent into the polymer is a slow diffusion process, the methoxy-methylation of polyamide articles having a considerable volume can prove too time-consuming from the viewpoint of its application on an industrial scale.